1. Field of the Invention
The present invention relates to a method and apparatus for decomposing organic fats and oils containing PCB (polychloro biphenyl) and for making them innoxious, and more particularly to a PCB decomposing method and apparatus in which PCB is mixed with water, an oxidizing agent and sodium carbonate, and decomposed under pressure at an elevated temperature.
2. Description of Related Art
Conventionally, several PCB decomposing methods have been known and their practical use has been studied by electric power companies, various manufactures and the like. However, only a few PCB decomposing methods have been completed and put to use for actual PCB decomposition treatments.
FIG. 10 shows a structure of a conventional PCB decomposing apparatus. In this PCB decomposing apparatus, a material to be treated which is contained in a transformer or the like is put in PCB extracting container 1. An organic solvent is fed from distilling tower 2 into the PCB extracting container 1 through transfer pump 4, and PCB is extracted with the organic solvent. The PCB extracted in the PCB extracting container 1 and the organic solvent are fed to the distilling column 2 through the transfer pump 3. In the distilling column 2, the organic solvent is distilled to separate the PCB. Then, the PCB circulates into the PCB extracting container 1 through the transfer pump 4.
The PCB extracted in the distilling tower 2 is mixed with water 10, aqueous hydrogen peroxide 11 and aqueous Na2CO3 solution 12. Then, the mixed water is pressurized to 250 ata by means of pressure pump 13 and passes through cooler 161 to be heated by the generated in reactor 16.
The mixed water containing the PCB cools the reactor 16, while the water mixture being heated to 300xc2x0 C., and enters the reactor 16 through start-up heater 15. In the reactor 16, reaction occurs after a residence time necessary for decomposition of the PCB. This reaction is exothermic. In order to keep the temperature at 380xc2x0 C., the reactor 16 is cooled by the cooler 161.
A reacted solution discharged from the reactor 16 is cooled to a temperature of 100xc2x0 C. or less by cooler 17. The deposited Na2CO3 is remelted. A pressure of the reacted solution is reduced to an atmospheric pressure by pressure reducing valve 18, and the solution is separated into CO2 and steam 20, and treated water 22 in steam separator or gas-liquid separator 21.
Exothermic reaction is carried out in the reactor 16 as expressed by the formula shown below.
PCB and solid Na2CO3 deposited upon heating react as shown in Formula (1) so that biphenyl, CO2 and NaCl are produced. The biphenyl further reacts with aqueous hydrogen peroxide solution so that CO2 and H2O are produced as shown in Formula (2). 
Formula 
In the reactor 16, a pipe is formed like a coil to increase a length thereof. Consequently, the residence time of the PCB in the reactor 16 is increased so that the PCB can sufficiently react, and its concentration can be reduced to the order of ppb (parts per billion).
In such a reactor of a pipe type, a solid deposited by Na2CO3 and PCB are not sufficiently stirred and a reaction rate tends to be low. Although the Na2CO3 is deposited by heating, a pipe is heated by a heater. Therefore, the Na2CO3 is deposited in the vicinity of an inside wall of the pipe. For this reason, the Na2CO3 sticks to the wall of the heated pipe. As a result, a heat transfer rate is reduced so that decomposition of the PCB is hindered. Furthermore, most of the reacted solution is water. Therefore, a large amount of water has to be treated, and an amount of water to be supplied is also large.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a method and apparatus for decomposing PCB capable of sufficiently stirring Na2CO3 and PCB and efficiently decomposing the PCB.
In order to attain the above-mentioned object, the present invention provides a PCB decomposing method comprising the steps of pressurizing PCB mixed with water to a predetermined pressure and heating the mixture, and then introducing the mixture into a container-shaped primary reactor, introducing aqueous Na2CO3 solution for PCB decomposition and blowing an oxidizing agent through a nozzle into the primary reactor, stirring the deposited Na2CO3 with the PCB by blowing of the nozzle to decompose the PCB into biphenyl containing no chlorine and NaCl, causing the biphenyl to react to the oxidizing agent, thereby decomposing the biphenyl into carbon dioxide and water, causing a fluid containing unreacted PCB to flow into a bent pipe in a secondary reactor, so as to increase a residence time of the fluid in the secondary reactor for a longer reaction time for the PCB and the Na2CO3, thereby promoting the decomposition of the PCB.
The present invention further provides a PCB decomposing apparatus comprising: a PCB extracting container for extracting PCB from a PCB-containing material with an organic solvent; a distilling tower for distilling an extracted solution introduced from the PCB extracting container, thereby separating the PCB from the organic solvent; a primary reactor including a container for receiving a PCB-containing fluid separated in the distilling tower, a nozzle for introducing aqueous sodium carbonate solution into the container for decomposing the PCB, and a nozzle for introducing an oxidizing agent; a secondary reactor having a conduit bent to increase a moving distance of the fluid and serving to further decompose unreacted PCB contained in the fluid introduced from the primary reactor in the conduit; and a gas-liquid separator for separating, from the fluid, carbon dioxide contained in the fluid introduced from the secondary reactor by reducing an internal pressure.
The PCB is mixed with water, and the mixture is pressurized to a predetermined pressure and preheated. The mixture is then introduced into the primary reactor. The primary reactor is formed by a container including a cooler instead of a conventional pipe. The heated mixture of PCB and water is introduced into the primary reactor, and an oxidizing agent, such as air, oxygen or the like, and aqueous Na2CO3 solution are injected into the primary reactor through the nozzle. Thus, mixing and reaction are carried out in the primary reactor.
The Na2CO3 deposited in the container and the PCB are stirred by jetting, through the nozzle, the oxidizing agent and the circulating water fetched from the container.
Chlorine molecules of the PCB react on surfaces of Na2CO3 particles deposited or precipitated with increased reaction temperatures. Consequently, the PCB is decomposed into NaCl and a biphenyl which does not substantially contain chlorine. Furthermore, the biphenyl reacts to the oxidizing agent and is decomposed into carbon dioxide and water. Much of the reaction between the Na2CO3 and PCB is solid-liquid reaction. Therefore, the reactor is fluidized by the circulating water so that a reaction rate can be increased.
From the solution flowing out of the primary reactor is removed the deposited or precipitated Na2CO3 having a large particle size by the liquid cyclone. Then, the solution is caused to react as it passes through the secondary reactor formed by a long pipe.
The secondary reactor formed by the pipe decomposes the PCB up to a ppb level. The treated water obtained after the reaction is concentrated by the film separator. The separated water is mixed with the PCB.